Hitherto, as a typical electro photographic photoreceptor for image forming purposes, a photoreceptor, for example, of the type having a photoreceptive layer containing inorganic photoconductive materials, such as selenium, zinc oxide, and cadmium sulfide, has been widely used.
Recently, various research and development efforts have been actively made for utilization of various kinds of organic photoconductive materials as materials for a photoreceptive layer of an electro-photographic photoreceptor.
For example, in Japanese Patent Publication No. 10496/1975 there is described an organic photoreceptor having a photoreceptive layer containing poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone. However, this photoreceptor is not very satisfactory in respect of sensitivity and durability. In order to overcome such deficiencies, attempts are being made to develop an organic photoreceptor having a photoreceptive layer in which charge generating and change transporting functions are separately pereformed by different materials, whereby high sensitivity and high durability requirements can be met.
In such electro-photographic photoreceptor of the so-called function partition type, materials capable of individually pereforming different functions can be selected from a wide range of groups of materials, and therefore an electrophotographic photoreceptor having any desired characteristics can be fabricated comparatively easily.
A large number of materials have hitherto been proposed as charge generating materials effective for use in such function partition type photoreceptor. For use of an inorganic material as such, in Japanese Patent Publication No. 16198/1968, for example, there is described the use of amorphous selenium, which material is used in combination with an organic charge transporting material.
Varieties of electro-photographic photoreceptors in which organic dyes or organic pigments are used as charge generating materials have also been proposed; for example, those having a photoreceptive layer containing a bisazo compound are already known through Japanese Provisional Patent Publication Nos. 37543/1972, 22834/1980, 79632/1979, and 116040/1981.
Now, the prior-art photoreceptors using aforesaid organic photoconductive materials are generally used as such for negative charging. The reason for this is that greater Hall mobility of charge is available in the case of negative charging, which fact is advantageous in respect of photosensitivity and otherwise. However, it has been found that the use of such negative charging photoreceptor involves the following problems.
One problem is that ozone is likely to be generated in the atmosphere in the course of negative charging by an electrifier, which will affect the environmental conditions. Another problem is that the development of a negative charging photoreceptor requires positive polarity toners, which are difficult to manufacture in view of the sequence of their triboelectric effect in relation to charge particles of ferromagnetic substances.
In view of these problems, it has been proposed to employ a photoreceptor using organic photoconductive materials, as such for positive charging. For example, when a charge transport layer is laid on a charge generating layer in forming a photoreceptor, the use of a material having a large electron transfer capability, such as trinitrofluorenone, for the charge transport layer has been proposed in order to efficiently nullify the positive charge on the surface of the photoreceptor. However, the use of such material is very unsuitable from the standpoint of pollution control, because the material is carcinogenic.
Another type of positive charging photoreceptor is disclosed in the specification of U.S. Pat. No. 3,615,414, which is such that thiapyrilium salt (a charge generating material) is used in combination with polycarbonate (a binder resin) so as to allow it to combine with the latter to form a eutectic complex. However, the use of this known type of photoreceptor involves considerable memory phenomenon. Another difficulty is that a ghost image is likely to occur. A photoreceptor in which phthalocyanine is used is disclosed in the specification of U.S. Pat. No. 3,357,989. However, phthalocyanine is liable to changes in its characteristics depending upon its crystalline form, which requires strict control of the crystalline form. In addition, this photoreceptor involves considerable memory phenomenon. Furthermore, its short wave sensitivity is low; therefore, the photoreceptor is considered unsuitable for use in copying machines which depend for its source of light upon visible light rays including short wave light.
As above noted, various attempts have been made in order to provide positive charging photoreceptors, but all the photoreceptors proposed so far involve problems yet to be solved in various respects, such as photosensitivity, memory, and/or pollution control.
In view of these difficulties, it is conceivable to employ for positive charging a photoreceptor having a lamination structure photoreceptive layer consisting of a charge generating layer containing a charge generating material which generates holes and electrons during illumination, as an upper layer (surface layer), and a charge transport layer containing a charge transporting material which has a hole transfer function, as a lower layer. It is also considered possible to employ for positive charging a photoreceptor having a photoreceptive layer of single-layer construction containing both of the charge generating and charge transporting materials.
In such photoreceptor for positive charging, if a charge generating material which has, for example, an electron attracting group in its molecular structure is used, it is considered that electron movement to nullify the positive charge on the surface of the photoreceptor is accelerated, it being thus possible to obtain high sensitivity characteristics.
However, in such positive charging photoreceptor, the layer containing the charge generating materials is formed as a surface layer, and accordingly the charge generating material, which is sensitive to light illumination, corona discharge, humidity, and more particularly to such external action as mechanical friction, is present in the vicinity of the surface layer; this may result in deterioration in the electro-photographic performance characteristics of the photoreceptor in the course of storage and image formation, which naturally means lower image quality.
In conventional negative charging photoreceptors having a charge transport layer as a surface layer, the charge generating layer is little liable to the influence of any such external action, and the charge transport layer serves to protect the lower layer, that is, the charge generating layer.
Contrary to this, in the case of positive charging photoreceptors, the surface layer which contains a charge generating material is subject to mechanical wear and damage due to external action and more particularly to developing and cleaning, with the result of various image defects, such as white spots and white lines, and also deterioration in electro-photographic performance characteristics, such as surface potential, sensitivity, memory, and residual potential.
Then, it may be conceivable to provide, for example, a thin protection layer comprised of a transparent insulating resin material to reinforce the layer containing the charge generating material, but in this case the trouble is that the charge generated during light illumination is blocked by the protective layer, with the result that the photoconductivity of the photoreceptor layer is lost.
It may also be conceivable to improve the resistance to wear and damage of the charge generating layer by increasing the thickness of the layer, but the trouble in this case is that the increased thickness leads to decreased sensitivity.
Therefore, the object of the invention is to provide an image forming method employing an electro-photographic photoreceptor which is advantageously constructed for positive charging purposes using organic photoconductive materials and which has high resistance to damage, high sensitivity, high degree of durability, and high resistance to ozone oxidation.